flex Definition

flex¹
verb flexed, flexing

1. To bend (a limb or joint).
2. To contract or tighten (a muscle) so as to bend a joint.

Idiom: flex one's muscles

To display the muscles of the arms and shoulders, etc by contracting them, prior to a trial of strength, etc.
Example: As he went into the boxing ring, he flexed his muscles

To refer to one's authority as a warning that it might be used.

Etymology: 16c: from Latin flectere to bend.

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flex²
noun

1. Flexible insulated electrical cable.

Etymology: Early 20c: from flexible.